The instrumentation tubing string assembly has been developed in connection with a specific technology--that of recovering oil from a subterranean formation or reservoir using a wellbore having a horizontal segment or leg. It will be described in the context of that type of well, although the assembly can be used in other well configurations as well.
The development in recent years of horizontal well technology has meant that an oil-bearing reservoir having a vertical thickness of, for example, 30 feet can be produced through a wellbore having a horizontal leg extending into the formation perhaps for 1500 feet. The "production interval" has been greatly extended as a result.
It is desirable for the operator of such a well to know whether all or only part of the horizontal leg of the wellbore is actually producing the oil. If only part of the horizontal leg is producing, it is desirable to know where that production is occurring and where it is not. If the fluid is only entering the wellbore over a 50 foot interval commencing 100 feet from the heel of the horizontal leg, the operator needs to know this. It might be possible to rework the well to increase the producing interval.
Establishing a reservoir pressure profile along the length of the horizontal leg will give an indication of where production is occurring; there is a formation pressure drawdown or reduction associated with those intervals where production is occurring.
One conventional way to establish such a pressure profile is to run a pressure logging tool through the leg. However this is expensive and timeconsuming to do and therefore can only be done from time to time. Another way is to position one or more pressure sensors, equipped with a battery and transmitting electronics, along one of the strings in the well. However, these assemblies are exposed to a wet environment (drilling mud, produced fluid and the like) and failures of downhole electronics and batteries are common.
Many of the horizontal wells in Alberta have been completed in formations containing heavy viscous oil--consequently they are involved in the practice of a variety of thermal processes. In general, steam is injected into the reservoir to warm the oil and improve its mobility. The heated oil is then produced.
It is desirable in connection with these wells to establish a temperature profile along the horizontal leg from time to time to monitor the heating process.
This can be done by supplying temperature sensors such as thermocouples or fibre optic devices at various locations along the production string in the horizontal leg and transmitting the measurements to ground surface through wiring or the fibre. However, the sensors are exposed to well fluids and failure is common.
With this background in mind, we set out to develop a system which can be characterized as follows:
positioning downhole pressure and temperature sensors along the horizontal leg, the sensors being capable of repeatedly measuring reservoir temperature and pressure in real time and transmitting an indication of these measurements through wiring or alternative means to data acquisition equipment at ground surface for recording, thereby eliminating the need for costly and intermittent temperature logging practices; PA1 housing the sensors in a secure dry environment to minimize failures; PA1 activating those sensors requiring actuating current, voltage, pressure or the like from ground surface to eliminate the need for downhole batteries; PA1 providing multiple pressure and temperature measuring sensors and correlating the location of the sensors to the desirable locations in an operator's well; PA1 avoiding the banding, clamping and strapping which characterize the prior art systems; and PA1 being able to retrieve the instrumentation assembly from the well without hindering the operation of the well. PA1 threading a bundle of sensor instrument lines, such as hard-wired temperature thermocouple sensors and the wiring for pressure sensors, through the bore of the tubing string. This was done at ground surface with the string uncoiled. The thermocouple sensors were staggered, as were the downhole or lower ends of the pressure sensor wiring, so that they would be located at predetermined measurement positions along the lower portion of the string, which would be located in the producing horizontal leg of the wellbore; PA1 the tubing string and the continuing contained instrument lines were then cut at each pressure-measuring point; PA1 a tubular steel sub having a port in its side wall was supplied for each cut point; PA1 at each cut point, a pressure sensor device was connected to its wiring and positioned in the bore of the tubing string adjacent the cut point; PA1 one set of cut ends of the instrument lines extending past the cut point was threaded through the bore of the sub and reconnected or spliced with the other set of cut ends; PA1 the pressure sensor was connected with the port in the sub. The sensor was adapted to contain or isolate formation fluid form the tubing string bore; PA1 the sub was then welded to the cut ends of the tubing string at the cut point; PA1 this process was repeated at each cut point along the string; PA1 the lower or toe end of the string was plugged or sealed; and PA1 the so-assembled continuous instrumentation tubing string was then coiled on a drum and run into a wellbore. As a consequence of practising this procedure, the following resulted: PA1 the sensors were now sealed in a dry bore; PA1 the sensors had means extending from the production interval up to ground surface, for transmitting signals indicative of the measurements made; PA1 the pressure sensors had access to the exterior reservoir pressure; PA1 multiple pressure and temperature sensors were now appropriately spaced along the length of the horizontal leg segment of the tubing string; and PA1 the entire assembly could be run into or out of the wellbore as a unit. PA1 (a) means for measuring a reservoir condition or characteristic, such as temperature or pressure or salinity, and producing a measurement signal indicative thereof, which could be an electrical signal (voltage, frequency or current), an optical signal, a pressure signal, an electromagnetic signal or a radio frequency signal; and PA1 (b) line means for transmitting the signal through the bore of the string to ground surface.